Place the stock pot on the stove over medium-high heat.
Pour in enough boiling water to cover the chicken wings by about 5 cm/2 inches. If you add too much water, it may make the broth too diluted. So it is better to make a concentrated broth which you can adjust with more water later, if necessary.
Add the spring onions, garlic, ginger, shitake mushrooms, and kombu.
When the pot nearly comes to a boil, turn the stove down to low heat.
Simmer the broth gently for 3 hours.
During this time, skim the broth regularly to remove any impurities. Also, maintain the water level by adding more boiling water as necessary.
After 3 hours, use a slotted spoon to remove all of the chicken wings and aromatics to a large bowl. You can discard these items or reserve the meat from the chicken wings to eat with the broth or another dish.
Strain the broth through a fine sieve into another large, clean stock pot.
To remove the excess fat from the broth, place a sheet of kitchen paper on the surface of the broth and quickly remove and discard it. Repeat this step a few more times. You want to leave a bit of fat in the broth for flavour.
Place the pot with the strained broth on the stove over medium-high heat.
Add the sugar, salt, soy sauce and mirin.
Taste for seasoning. You may need to add more salt or soy sauce, and/or more boiling water if the broth is too concentrated.